Nice setup. I did find this article in the learning center according to it you don't need to stress much over them: https://www.backyardchickens.com/articles/female-ducks-with-their-babies.74650/
Hope this information is helpful, and here is a link to all of them...
@WVduckchick might have a few pointers for you for my broody chicken get that way once they see a large enough clutch, so I suspect you are on the right track of thinking clutch size matters.
Photos of your housing for them would be a big help.